Pipeline arbitration

ABSTRACT

A method includes receiving, by a first stage in a pipeline, a first transaction from a previous stage in pipeline; in response to first transaction comprising a high priority transaction, processing high priority transaction by sending high priority transaction to a buffer; receiving a second transaction from previous stage; in response to second transaction comprising a low priority transaction, processing low priority transaction by monitoring a full signal from buffer while sending low priority transaction to buffer; in response to full signal asserted and no high priority transaction being available from previous stage, pausing processing of low priority transaction; in response to full signal asserted and a high priority transaction being available from previous stage, stopping processing of low priority transaction and processing high priority transaction; and in response to full signal being de-asserted, processing low priority transaction by sending low priority transaction to buffer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/852,461, which was filed May 24, 2019, is titled“Cache And Cache Controller For Processor,” and is hereby incorporatedherein by reference in its entirety.

BACKGROUND

Some memory systems include a multi-level cache system, in which ahierarchy of memories (e.g., caches) provides varying access speeds tocache data. A first level (L1) cache is closely coupled to a centralprocessing unit (CPU) core and provides the CPU core with faster access(e.g., relative to main memory) to cache data. A second level (L2) cacheis also coupled to the CPU core and, in some examples, is larger andthus holds more data than the L1 cache, although the L2 cache providesrelatively slower access to cache data than the L1 cache. Additionalmemory levels of the hierarchy are possible.

SUMMARY

In accordance with at least one example of the disclosure, a methodincludes receiving a first request to allocate a line in an N-way setassociative cache and, in response to a cache coherence state of a wayindicating that a cache line stored in the way is invalid, allocatingthe way for the first request. The method also includes, in response tono ways in the set having a cache coherence state indicating that thecache line stored in the way is invalid, randomly selecting one of theways in the set. The method also includes, in response to a cachecoherence state of the selected way indicating that another request isnot pending for the selected way, allocating the selected way for thefirst request.

In accordance with another example of the disclosure, a method includesreceiving a first request to allocate a line in an N-way set associativecache and, in response to a cache coherence state of a way indicatingthat a cache line stored in the way is invalid, allocating the way forthe first request. The method also includes, in response to no ways inthe set having a cache coherence state indicating that the cache linestored in the way is invalid, creating a masked subset of ways in theset by masking any way having a cache coherence state indicating thatanother request is pending for the way, randomly selecting one of theways in the masked subset, and allocating the selected way for the firstrequest.

In accordance with yet another example of the disclosure, a level two(L2) cache subsystem includes a L2 cache configured as an N-way setassociative cache and a L2 controller configured to receive a firstrequest to allocate a line in the L2 cache and, in response to a cachecoherence state of a way indicating that a cache line stored in the wayis invalid, allocate the way for the first request. The L2 controller isalso configured to, in response to no ways in the set having a cachecoherence state indicating that the cache line stored in the way isinvalid, randomly select one of the ways in the set. The L2 controlleris also configured to, in response to a cache coherence state of theselected way indicating that another request is not pending for theselected way, allocate the selected way for the first request.

In accordance with at least one example of the disclosure, a methodincludes receiving, by a first stage in a pipeline, a first transactionfrom a previous stage in the pipeline; determining whether the firsttransaction comprises a high priority transaction or a low prioritytransaction; in response to the first transaction comprising a highpriority transaction, processing the high priority transaction bysending the high priority transaction to an output buffer; receiving asecond transaction from the previous stage; and determining whether thesecond transaction comprises a high priority transaction or a lowpriority transaction. In response to the second transaction comprising alow priority transaction, the method includes processing the lowpriority transaction by monitoring a full signal from the output bufferwhile sending the low priority transaction to the output buffer; inresponse to the full signal being asserted and no high prioritytransaction being available from the previous stage, pausing processingof the low priority transaction; in response to the full signal beingasserted and a high priority transaction being available from theprevious stage, stopping processing of the low priority transaction andprocessing the high priority transaction; and in response to the fullsignal being de-asserted, processing the low priority transaction bysending the low priority transaction to the output buffer.

In accordance with another example of the disclosure, a method includesreceiving, by a first stage in a pipeline, a first transaction from aprevious stage in a pipeline; determining whether the first transactioncomprises a high priority transaction, a medium priority transaction, ora low priority transaction; in response to the first transactioncomprising a high priority transaction, processing the high prioritytransaction by sending the high priority transaction to an outputbuffer. The method also includes receiving a second transaction from theprevious stage; determining whether the second transaction comprises amedium priority transaction or a low priority transaction. In responseto the second transaction comprising a medium priority transaction, themethod includes processing the medium priority transaction by monitoringa full signal from the output buffer while sending the medium prioritytransaction to the output buffer; in response to the full signal beingasserted and no high priority transaction being available from theprevious stage, pausing processing of the medium priority transaction;in response to the full signal being asserted and a high prioritytransaction being available from the previous stage, stopping processingof the medium priority transaction and processing the high prioritytransaction; and in response to the full signal being de-asserted,processing the medium priority transaction by sending the mediumpriority transaction to the output buffer. The method also includes, inresponse to the second transaction comprising a low prioritytransaction, processing the low priority transaction by monitoring thefull signal from the output buffer while sending the low prioritytransaction to the output buffer; in response to the full signal beingasserted and no high or medium priority transaction being available fromthe previous stage, pausing processing of the low priority transaction;in response to the full signal being asserted and a high or mediumpriority transaction being available from the previous stage, stoppingprocessing of the low priority transaction and processing the high ormedium priority transaction; and in response to the full signal beingde-asserted, processing the low priority transaction by sending themedium priority transaction to the output buffer.

In accordance with yet another example of the disclosure, a methodincludes level two (L2) cache subsystem, comprising a L2 pipeline and astate machine in the L2 pipeline. The state machine is configured toreceive a first transaction from an input buffer coupled to a previousstage in the L2 pipeline; determine whether the first transactioncomprises a high priority transaction, a medium priority transaction, ora low priority transaction; and in response to the first transactioncomprising a high priority transaction, process the high prioritytransaction by sending the high priority transaction to an outputbuffer. The state machine is also configured to receive a secondtransaction from the input buffer; determine whether the secondtransaction comprises a medium priority transaction or a low prioritytransaction; and, in response to the second transaction comprising amedium priority transaction, process the medium priority transaction.When the state machine processes the medium priority transaction, thestate machine is further configured to monitor a full signal from theoutput buffer while the medium priority transaction is sent to theoutput buffer; in response to the full signal being asserted and no highpriority transaction being available from the input buffer, pauseprocessing of the medium priority transaction; in response to the fullsignal being asserted and a high priority transaction being availablefrom the input buffer, stop processing of the medium prioritytransaction and process the high priority transaction; and in responseto the full signal being de-asserted, process the medium prioritytransaction by sending the medium priority transaction to the outputbuffer. The state machine is also configured to in response to thesecond transaction comprising a low priority transaction, process thelow priority transaction. When the state machine processes the lowpriority transaction, the state machine is further configured to monitorthe full signal from the output buffer while the low prioritytransaction is sent to the output buffer; in response to the full signalbeing asserted and no high or medium priority transaction beingavailable from the input buffer, pause processing of the low prioritytransaction; in response to the full signal being asserted and a high ormedium priority transaction being available from the input buffer, stopprocessing of the low priority transaction and process the high ormedium priority transaction; and, in response to the full signal beingde-asserted, process the low priority transaction by sending the mediumpriority transaction to the output buffer.

In accordance with at least one example of the disclosure, an apparatusincludes a CPU core, a first cache subsystem coupled to the CPU core,and a second memory coupled to the cache subsystem. The first cachesubsystem includes a configuration register, a first memory, and acontroller. The controller is configured to: receive a request directedto an address in the second memory and, in response to the configurationregister having a first value, operate in a non-caching mode. In thenon-caching mode, the controller is configured to provide the request tothe second memory without caching data returned by the request in thefirst memory. In response to the configuration register having a secondvalue, the controller is configured to operate in a caching mode. In thecaching mode the controller is configured to provide the request to thesecond memory and cache data returned by the request in the firstmemory.

In accordance with another example of the disclosure, a method includesreceiving, by a level two (L2) controller comprising a configurationregister, a request directed to an address in a level three (L3) memory;and, in response to the configuration register having a first value,operating the L2 controller in a non-caching mode by providing therequest to the L3 memory and not caching data returned by the request ina L2 cache. In response to the configuration register having a secondvalue, the method includes operating the L2 controller in a caching modeby providing the request to the second memory and caching data returnedby the request in the L2 cache.

In accordance with yet another example of the disclosure, a level two(L2) cache subsystem includes a configuration register, a first memory,and a L2 controller. The L2 controller is configured to receive arequest directed to an address in a second memory coupled to the L2cache subsystem and, in response to the configuration register having afirst value, operate in a non-caching mode. In the non-caching mode theL2 controller is configured to provide the request to the second memorywithout caching data returned by the request in the first memory. Inresponse to the configuration register having a second value, the L2controller operates in a caching mode. In the caching mode, the L2controller is configured to provide the request to the second memory andcache data returned by the request in the first memory.

In accordance with at least one example of the disclosure, an apparatusincludes first CPU and second CPU cores, a L1 cache subsystem coupled tothe first CPU core and comprising a L1 controller, and a L2 cachesubsystem coupled to the L1 cache subsystem and to the second CPU core.The L2 cache subsystem includes a L2 memory and a L2 controllerconfigured to operate in an aliased mode in response to a value in amemory map control register being asserted. In the aliased mode, the L2controller receives a first request from the first CPU core directed toa virtual address in the L2 memory, receives a second request from thesecond CPU core directed to the virtual address in the L2 memory,directs the first request to a physical address A in the L2 memory, anddirects the second request to a physical address B in the L2 memory.

In accordance with at least one example of the disclosure, a methodincludes operating a level two (L2) controller of a L2 cache subsystemin an aliased mode in response to a memory map control register valuebeing asserted. Operating the L2 controller in the aliased mode furthercomprises receiving a first request from a first CPU core directed to avirtual address in a L2 memory of the L2 cache subsystem, receiving asecond request from a second CPU core directed to the virtual address inthe L2 memory, directing the first request to a physical address A inthe L2 memory, and directing the second request to a physical address Bin the L2 memory.

In accordance with at least one example of the disclosure, a methodincludes receiving, by a level two (L2) controller, a write request foran address that is not allocated as a cache line in a L2 cache. Thewrite request specifies write data. The method also includes generating,by the L2 controller, a read request for the address; reserving, by theL2 controller, an entry in a register file for read data returned inresponse to the read request; updating, by the L2 controller, a datafield of the entry with the write data; updating, by the L2 controller,an enable field of the entry associated with the write data; andreceiving, by the L2 controller, the read data and merging the read datainto the data field of the entry.

In accordance with another example of the disclosure, a level two (L2)cache subsystem includes a L2 cache, a register file having an entry,and a L2 controller. The L2 controller is configured to receive a writerequest for an address that is not allocated as a cache line in the L2cache, the write request comprising write data; generate a read requestfor the address; reserve the entry in the register file for read datareturned in response to the read request; update a data field of theentry with the write data; update an enable field of the entryassociated with the write data; and receive the read data and merge theread data into the data field of the entry.

In accordance with yet another example of the disclosure, an apparatusincludes a central processing unit (CPU) core and a level one (L1) cachesubsystem coupled to the CPU core. The L1 cache subsystem includes a L1cache, and a L1 controller. The apparatus also includes a level two (L2)cache subsystem coupled to the L1 cache subsystem. The L2 cachesubsystem includes a L2 cache, a register file having an entry, and a L2controller. The L2 controller is configured to receive a write requestfor an address that is not allocated as a cache line in the L2 cache,the write request including write data; generate a read request for theaddress; reserve the entry in the register file for read data returnedin response to the read request; update a data field of the entry withthe write data; update an enable field of the entry associated with thewrite data; and receive the read data and merge the read data into thedata field of the entry.

In accordance with at least one example of the disclosure, a methodincludes receiving, by a L2 controller, a request to perform a globaloperation on a L2 cache and preventing new blocking transactions fromentering a pipeline coupled to the L2 cache while permitting newnon-blocking transactions to enter the pipeline. Blocking transactionsinclude read transactions and non-victim write transactions.Non-blocking transactions include response transactions, snooptransactions, and victim transactions. The method further includes, inresponse to an indication that the pipeline does not contain any pendingblocking transactions, preventing new snoop transactions from enteringthe pipeline while permitting new response transactions and victimtransactions to enter the pipeline; in response to an indication thatthe pipeline does not contain any pending snoop transactions,preventing, all new transactions from entering the pipeline; and, inresponse to an indication that the pipeline does not contain any pendingtransactions, performing the global operation on the L2 cache.

In accordance with another example of the disclosure, an apparatusincludes a central processing unit (CPU) core and a level one (L1) cachesubsystem coupled to the CPU core. The L1 cache subsystem includes a L1cache, a L1 controller, and a level two (L2) cache subsystem coupled tothe L1 cache subsystem. The L2 cache subsystem includes a L2 cache and aL2 controller. The L2 controller is configured to receive a request toperform a global operation on the L2 cache and prevent new blockingtransactions from entering a pipeline coupled to the L2 cache and permitnew non-blocking transactions to enter the pipeline. Blockingtransactions include read transactions and non-victim writetransactions. Non-blocking transactions include response transactions,snoop transactions, and victim transactions. The L2 controller isfurther configured to, in response to an indication that the pipelinedoes not contain any pending blocking transactions, prevent new snooptransactions from entering the pipeline and permit new responsetransactions and victim transactions to enter the pipeline; in responseto an indication that the pipeline does not contain any pending snooptransactions, prevent all new transactions from entering the pipeline;and, in response to an indication that the pipeline does not contain anypending transactions, perform the global operation on the L2 cache.

In accordance with yet another example of the disclosure, a level two(L2) cache subsystem includes a L2 cache and a L2 controller. The L2controller is configured to receive a request to perform a globaloperation on the L2 cache and prevent new blocking transactions fromentering a pipeline coupled to the L2 cache and permit new non-blockingtransactions to enter the pipeline. Blocking transactions include readtransactions and non-victim write transactions. Non-blockingtransactions include response transactions, snoop transactions, andvictim transactions. The L2 controller is further configured to, inresponse to an indication that the pipeline does not contain any pendingblocking transactions, prevent new snoop transactions from entering thepipeline and permit new response transactions and victim transactions toenter the pipeline; in response to an indication that the pipeline doesnot contain any pending snoop transactions, prevent all new transactionsfrom entering the pipeline; and, in response to an indication that thepipeline does not contain any pending transactions, perform the globaloperation on the L2 cache.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows a block diagram of a multi-level cache system in accordancewith various examples;

FIG. 2 shows another block diagram of a multi-level cache system inaccordance with various examples;

FIG. 3 shows another block diagram of a multi-level cache systemincluding level one (L1) main and victim caches in accordance withvarious examples;

FIG. 4 shows a block diagram of a multi-stage pipeline in accordancewith various examples;

FIG. 5 shows a block diagram of a P4 stage of the pipeline includingtransaction arbitration in accordance with various examples;

FIG. 6 is a flow chart of a method for transaction arbitration inaccordance with various examples;

FIG. 7 is a flow chart of a method for stalling a cache pipeline toperform a global operation in accordance with various examples;

FIG. 8a shows an example of improving cache allocation in response to awrite allocate in accordance with various examples;

FIG. 8b shows an example of register files used in FIG. 8a in accordancewith various examples;

FIG. 9 shows a flow chart of a method of improving cache allocation inresponse to a write allocate in accordance with various examples;

FIG. 10 shows an example and flow chart of a mask-based way selectionpolicy using a random number generator in accordance with variousexamples;

FIG. 11 shows a flow chart of an alternate way selection policy using arandom number generator in accordance with various examples;

FIG. 12 shows a flow chart of operating a cache controller in cachingand non-caching modes in accordance with various examples;

FIG. 13 shows an example of aliased and un-aliased memory operations inaccordance with various examples; and

FIG. 14 shows a flow chart of a method for performing aliased andun-aliased memory operations in accordance with various examples.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a system 100 in accordance with anexample of this disclosure. The example system 100 includes multiple CPUcores 102 a-102 n. Each CPU core 102 a-102 n is coupled to a dedicatedL1 cache 104 a-104 n and a dedicated L2 cache 106 a-106 n. The L2 caches106 a-106 n are, in turn, coupled to a shared third level (L3) cache 108and a shared main memory 110 (e.g., double data rate (DDR) random-accessmemory (RAM)). In other examples, a single CPU core 102 is coupled to aL1 cache 104, a L2 cache 106, a L3 cache 108, and main memory 110.

In some examples, the CPU cores 102 a-102 n include a register file, aninteger arithmetic logic unit, an integer multiplier, and program flowcontrol units. In an example, the L1 caches 104 a-104 n associated witheach CPU core 102 a-102 n include a separate level one program cache(L1P) and level one data cache (L1D). The L2 caches 106 a-106 n arecombined instruction/data caches that hold both instructions and data.In certain examples, a CPU core 102 a and its associated L1 cache 104 aand L2 cache 106 a are formed on a single integrated circuit.

The CPU cores 102 a-102 n operate under program control to perform dataprocessing operations upon data. Instructions are fetched beforedecoding and execution. In the example of FIG. 1, L1P of the L1 cache104 a-104 n stores instructions used by the CPU cores 102 a-102 n. A CPUcore 102 first attempts to access any instruction from L1P of the L1cache 104. L1D of the L1 cache 104 stores data used by the CPU core 102.The CPU core 102 first attempts to access any required data from L1cache 104. The two L1 caches 104 (L1P and L1D) are backed by the L2cache 106, which is a unified cache (e.g., includes both data andinstructions). In the event of a cache miss to the L1 cache 104, therequested instruction or data is sought from L2 cache 106. If therequested instruction or data is stored in the L2 cache 106, then it issupplied to the requesting L1 cache 104 for supply to the CPU core 102.The requested instruction or data is simultaneously supplied to both therequesting cache and CPU core 102 to speed use.

The unified L2 cache 106 is further coupled to a third level (L3) cache108, which is shared by the L2 caches 106 a-106 n in the example ofFIG. 1. The L3 cache 108 is in turn coupled to a main memory 110. Aswill be explained in further detail below, memory controllers facilitatecommunication between various ones of the CPU cores 102, the L1 caches104, the L2 caches 106, the L3 cache 108, and the main memory 110. Thememory controller(s) handle memory centric functions such as cacheabiltydetermination, cache coherency implementation, error detection andcorrection, address translation and the like. In the example of FIG. 1,the CPU cores 102 are part of a multiprocessor system, and thus thememory controllers also handle data transfer between CPU cores 102 andmaintain cache coherence among CPU cores 102. In other examples, thesystem 100 includes only a single CPU core 102 along with its associatedL1 cache 104 and L2 cache 106.

FIG. 2 shows a block diagram of a system 200 in accordance with examplesof this disclosure. Certain elements of the system 200 are similar tothose described above with respect to FIG. 1, although shown in greaterdetail. For example, a CPU core 202 is similar to the CPU core 102described above. The L1 cache 104 subsystem described above is depictedas L1D 204 and L1P 205. The L2 cache 106 described above is shown hereas L2 cache subsystem 206. An L3 cache 208 is similar to the L3 cache108 described above. The system 200 also includes a streaming engine 210coupled to the L2 cache subsystem 206. The system 200 also includes amemory management unit (MMU) 207 coupled to the L2 cache subsystem 206.

The L2 cache subsystem 206 includes L2 tag ram 212, L2 coherence (e.g.,Modified, Exclusive, Shared, Invalid (“MESI”)) data memory 214, shadowL1 tag ram 216, and L1 coherence (e.g., MESI) data memory 218. Each ofthe blocks 212, 214, 216, 218 are alternately referred to as a memory ora RAM. The L2 cache subsystem 206 also includes tag ram error correctingcode (ECC) data memory 220. In an example, the ECC data memory 220 ismaintained for each of the memories 212, 214, 216, 218.

The L2 cache subsystem 206 includes L2 controller 222, the functionalityof which will be described in further detail below. In the example ofFIG. 2, the L2 cache subsystem 206 is coupled to memory (e.g., L2 SRAM224) including four banks 224 a-224 d. An interface 230 performs dataarbitration functions and generally coordinates data transmissionbetween the L2 cache subsystem 206 and the L2 SRAM 224, while an ECCblock 226 performs error correction functions. The L2 cache subsystem206 includes one or more control or configuration registers 228.

In the example of FIG. 2, the L2 SRAM is depicted as four banks 224a-224 d. However, in other examples, the L2 SRAM includes more or fewerbanks, including being implemented as a single bank. The L2 SRAM 224serves as the L2 cache and is alternately referred to herein as L2 cache224.

The L2 tag ram 212 includes a list of the physical addresses whosecontents (e.g., data or program instructions) have been cached to the L2cache 224. In an example, an address translator translates virtualaddresses to physical addresses. In one example, the address translatorgenerates the physical address directly from the virtual address. Forexample, the lower n bits of the virtual address are used as the leastsignificant n bits of the physical address, with the most significantbits of the physical address (above the lower n bits) being generatedbased on a set of tables configured in main memory. In this example, theL2 cache 224 is addressable using physical addresses. In certainexamples, a hit/miss indicator from a tag ram 212 look-up is stored in amemory.

The L2 MESI memory 214 maintains coherence data to implement full MESIcoherence with L2 SRAM 224, external shared memories, and data cached inL2 cache from other places in the system 200. The functionalities ofsystem 200 coherence are explained in further detail below.

The L2 cache subsystem 206 also tracks or shadows L1D tags in the L1Dshadow tag ram 216 and L1D MESI memory 218. The tag ram ECC data 220provides error detection and correction for the tag memories and,additionally, for one or both of the L2 MESI memory 214 and the L1D MESImemory 218. The L2 cache controller 222 controls the operations of theL2 cache subsystem 206, including handling coherency operations bothinternal to the L2 cache subsystem 206 and among the other components ofthe system 200.

FIG. 3 shows a block diagram of a system 300 that demonstrates variousfeatures of cache coherence implemented in accordance with examples ofthis disclosure. The system 300 contains elements similar to thosedescribed above with respect to FIGS. 1 and 2. For example, the CPU core302 is similar to the CPU cores 102, 202. FIG. 3 also includes a L1cache subsystem 304, a L2 cache subsystem 306, and an L3 cache subsystem308. The L1 cache subsystem 304 includes a L1 controller 310 coupled toL1 SRAM 312. The L1 controller 310 is also coupled to a L1 main cache314 and a L1 victim cache 316, which are explained in further detailbelow. In some examples, the L1 main and victim caches 314, 316implement the functionality of L1D 204 and/or L1P 205.

The L1 controller 310 is coupled to a L2 controller 320 of the L2 cachesubsystem 306. The L2 controller 320 also couples to L2 SRAM 322. The L2controller 320 couples to a L2 cache 324 and to a shadow of the L1 maincache 326 as well as a shadow of the L1 victim cache 328. L2 cache 324and L2 SRAM 322 are shown separately for ease of discussion, althoughmay be implemented physically together (e.g., as part of L2 SRAM 224,including in a banked configuration, as described above. Similarly, theshadow L1 main cache 326 and the shadow L1 victim cache 328 may beimplemented physically together, and are similar to the L1D shadow tagram 216 and the L1D MESI 218, described above. The L2 controller 320 isalso coupled to a L3 controller 309 of the L3 cache subsystem 308. L3cache and main memory (e.g., DDR 110 described above) are not shown forsimplicity.

Cache coherence is a technique that allows data and program caches, aswell as different requestors (including requestors that do not havecaches) to determine the most current data value for a given address inmemory. Cache coherence enables this coherent data value to bedetermined by observers (e.g., a cache or requestor that issues commandsto read a given memory location) present in the system 300. Certainexamples of this disclosure refer to an exemplary MESI coherence scheme,in which a cache line is set to one of four cache coherence states:modified, exclusive, shared, or invalid. Other examples of thisdisclosure refer to a subset of the MESI coherence scheme, while stillother examples include more coherence states than the MESI coherencescheme. Regardless of the coherence scheme, cache coherence states for agiven cache line are stored in, for example, the L2 MESI memory 214described above.

A cache line having a cache coherence state of modified indicates thatthe cache line is modified with respect to main memory (e.g., DDR 110),and the cache line is held exclusively in the current cache (e.g., theL2 cache 324). A modified cache coherence state also indicates that thecache line is explicitly not present in any other caches (e.g., L1 or L3caches).

A cache line having a cache coherence state of exclusive indicates thatthe cache line is not modified with respect to main memory (e.g., DDR110), but the cache line is held exclusively in the current cache (e.g.,the L2 cache 324). An exclusive cache coherence state also indicatesthat the cache line is explicitly not present in any other caches (e.g.,L1 or L3 caches).

A cache line having a cache coherence state of shared indicates that thecache line is not modified with respect to main memory (e.g., DDR 110).A shared cache state also indicates that the cache line may be presentin multiple caches (e.g., caches in addition to the L2 cache 324).

A cache line having a cache coherence state of invalid indicates thatthe cache line is not present in the cache (e.g., the L2 cache 324).

Examples of this disclosure leverage hardware techniques, control logic,and/or state information to implement a coherent system. Each observercan issue read requests—and certain observers are able to issue writerequests—to memory locations that are marked shareable. Caches inparticular can also have snoop requests issued to them, requiring theircache state to be read, returned, or even updated, depending on the typeof the snoop operation. In the exemplary multi-level cache hierarchydescribed above, the L2 cache subsystem 306 is configured to both sendand receive snoop operations. The L1 cache subsystem 304 receives snoopoperations, but does not send snoop operations. The L3 cache subsystem308 sends snoop operations, but does not receive snoop operations. Inexamples of this disclosure, the L2 cache controller 320 maintains stateinformation (e.g., in the form of hardware buffers, memories, and logic)to additionally track the state of coherent cache lines present in boththe L1 main cache 314 and the L1 victim cache 316. Tracking the state ofcoherent cache lines enables the implementation of a coherent hardwarecache system.

Examples of this disclosure refer to various types of coherenttransactions, including read transactions, write transactions, snooptransactions, victim transactions, and cache maintenance operations(CMO). These transactions are at times referred to as reads, writes,snoops, victims, and CMOs, respectively.

Reads return the current value for a given address, whether that valueis stored at the endpoint (e.g., DDR 110), or in one of the caches inthe coherent system 300. Writes update the current value for a givenaddress, and invalidate other copies for the given address stored incaches in the coherent system 300. Snoops read or invalidate (or both)copies of data stored in caches. Snoops are initiated from anumerically-higher level of the hierarchy to a cache at the next,numerically-lower level of the hierarchy (e.g., from the L2 controller320 to the L1 controller 310), and are able be further propagated toeven lower levels of the hierarchy as needed. Victims are initiated froma numerically-lower level cache in the hierarchy to the next,numerically-higher level of the cache hierarchy (e.g., from the L1controller 310 to the L2 controller 320). Victims transfer modified datato the next level of the hierarchy. In some cases, victims are furtherpropagated to numerically-higher levels of the cache hierarchy (e.g., ifthe L2 controller 310 sends a victim to the L2 controller 320 for anaddress in the DDR 110, and the line is not present in the L2 cache 324,the L2 controller 320 forwards the victim to the L3 controller 309).Finally, CMOs cause an action to be taken in one of the caches for agiven address.

Still referring to FIG. 3, in one example, the L1 main cache 314 is adirect mapped cache that services read and write hits and snoops. The L1main cache 314 also keeps track of cache coherence state information(e.g., MESI state) for its cache lines. In an example, the L1 main cache314 is a read-allocate cache. Thus, writes that miss the L1 main cache314 are sent to L2 cache subsystem 306 without allocating space in theL1 main cache 314. In the example where the L1 main cache 314 is directmapped, when a new allocation takes place in the L1 main cache 314, thecurrent line in the set is moved to the L1 victim cache 316, regardlessof whether the line is clean (e.g., unmodified) or dirty (e.g.,modified).

In an example, the L1 victim cache 316 is a fully associative cache thatholds cache lines that have been removed from the L1 main cache 314, forexample due to replacement. The L1 victim cache 316 holds both clean anddirty lines. The L1 victim cache 316 services read and write hits andsnoops. The L1 victim cache 316 also keeps track of cache coherencestate information (e.g., MESI state) for its cache lines. When a cacheline in the modified state is replaced from the L1 victim cache 316,that cache line is sent to the L2 cache subsystem 306 as a victim.

As explained above, the L2 cache subsystem 306 includes a unified L2cache 324 that is used to service requests from multiple requestortypes, including L1D and L1P (through the L1 controller 310), thestreaming engine 210, a memory management unit (MMU 207), and the L3cache (through the L3 controller 309). In an example, the L2 cache 324is non-inclusive with the L1 cache subsystem 304, which means that theL2 cache 324 is not required to include all cache lines stored in the L1caches 314, 316, but that some lines may be cached in both levels.Continuing this example, the L2 cache 324 is also non-exclusive, whichmeans that cache lines are not explicitly prevented from being cached inboth the L1 and L2 caches 314, 316, 324. For example, due to allocationand random replacement, cache lines may be present in one, both, orneither of the L1 and L2 caches. The combination of non-inclusive andnon-exclusive cache policies enables the L2 controller 320 to manage itscache contents without requiring the L1 controller 310 to invalidate orremove cache lines. This simplifies processing in the L2 cache subsystem306 and enables increased performance for the CPU core 302 by allowingcritical data to remain cached in the L1 cache subsystem 304 even if ithas been evicted from the L2 cache 324.

In accordance with examples of this disclosure, the L2 cache subsystem306 includes a control pipeline that processes transactions of differenttypes. In certain examples in this disclosure, transactions areclassified as blocking or non-blocking, for example based on whether areceiving device is permitted to delay or stall the transaction.Examples of blocking transactions include read and write requests andinstruction fetches. Examples of non-blocking transactions includevictims, snoops, and responses to read and/or write requests. Stillreferring to FIG. 3, the L2 controller 320 described herein combinesboth local coherence (e.g., handling requests targeting its local L2SRAM 322 as an endpoint) and external coherence (e.g., handling requeststargeting external memories, such as L3 SRAM (not shown for simplicity)or DDR 110 as endpoints). An endpoint refers to a memory target such asL2 SRAM 322 or DDR 110 that resides at a particular location on thechip, is acted upon directly by a single controller and/or interface,and may be cached at various levels of a coherent cache hierarchy, suchas depicted in FIG. 3. A master (e.g., a hardware component, circuitry,or the like) refers to a requestor that issues read and write accessesto an endpoint. In some examples, a master stores the results of theseread and write accesses in a cache, although the master does notnecessarily store such results in a cache.

In an example, an endpoint (e.g., the L3 cache subsystem 308 for cachetransactions originating from the L2 controller 320, and the L1 cachesubsystem 304 for snoop transactions originating from the L2 controller320) will not stall non-blocking transactions behind another blockingtransaction. As a result, non-blocking transactions are guaranteed to beconsumed by the endpoint. Blocking transactions, however, can be stalledindefinitely by the endpoint. The L2 controller 320 sends both blockingand non-blocking transactions to both the L3 controller 309 and the L1controller 310. If the L2 controller 320 has a blocking transaction tobe sent out, but that is stalled, then a pipeline controller (e.g.,arbitration logic) ensures that a non-blocking transaction can bypassthe stalled blocking transaction and be sent out to the endpoint. As oneexample, the L2 pipeline is filled with reads from the streaming engine205, which are blocking transactions. The L3 controller 309 is able tostall such streaming reads. However, if the L1 controller 310 needs tosend a victim to the L2 controller, or if the L2 controller 320 needs torespond to a snoop from the L3 controller 309, examples of thisdisclosure permit such non-blocking transactions to be sent out throughthe same control pipeline.

FIG. 4 shows a pipeline 400 of the L2 cache subsystem 306 in accordancewith examples of this disclosure. Certain examples of this disclosurepertain particularly to transaction arbitration carried out in pipestage P4 428. However, the pipeline 400 is described below foradditional context and clarity. The pipeline 400 receives transactionsfrom various masters, such as program memory controller 402 (e.g., PMCor L1P 205), data memory controller 404 (e.g., DMC or L1D 204), astreaming engine 406 (e.g., SE 210), a multicore shared memorycontroller 408 (e.g., MSMC or L3 controller 309), and a memorymanagement unit 410 (e.g., MMU 207). A plurality of FIFOs 412 containdifferent types of transactions from the various masters 402, 404, 406,408, 410, while a resource allocation unit (RAU) 414, 416, 418arbitrates transactions from each requestor, for example based on theparticular type of requestor and the type of transactions that canoriginate from that requestor. For purposes of this disclosure,transactions are classified as blocking and non-blocking.

The RAU stages 414, 416, 418 arbitrate among different transactiontypes, which have certain characteristics. For example, blocking readsand writes include data loads and stores, code fetches, and SE 406reads. These blocking transactions can stall behind a non-blockingtransaction or a response. Another example includes non-blocking writes,which include DMC 404 victims (either from a local CPU core or from adifferent CPU core cached by the DMC 404). These types of transactionsare arbitrated with other non-blocking and response transactions basedon coherency rules. Another example includes non-blocking snoops, whichare snoops from MSMC 408 that are arbitrated with other non-blocking andresponse transactions based on coherency rules. Another example includesresponses, such as to a read or cache line allocate transaction sent outto MSMC 408, or for a snoop sent to DMC 404. In both case, responses arearbitrated with other non-blocking and response transactions based oncoherency rules. Finally, DMA transactions are possible, which aregenerally allowed to stall behind other non-blocking or blockingtransactions.

Not all requestors originate all these types of transactions. Forexample, DMC 404 can originate blocking reads, blocking writes,non-blocking writes (e.g., DMC 404 victims), non-blocking snoopresponses, and non-blocking DMA response (e.g., for L1D 204 SRAM). Forthe DMC 404, non-blocking transactions win arbitration over blockingtransactions. Between the various non-blocking transactions,non-blocking commands are processed in the order that they arrive. DMAresponses are for accesses to L1D 204 SRAM and do not necessarily followany command ordering.

An example PMC 402 can originate only blocking reads. In one example,reads from PMC 402 are processed in order.

An example SE 406 can originate blocking reads and CMOs. In one example,reads and CMO accesses from SE 406 are processed in order.

An example MMU 410 can originate only blocking reads. In one example,reads from MMU 410 are processed in order.

Finally, an example MSMC 408 can originate blocking DMA reads, blockingDMA writes, non-blocking writes (e.g., L1D 204 victims from another CPUcore), non-blocking snoops, and non-blocking read responses. For MSMC408, non-blocking transactions win arbitration over blockingtransactions. Arbitration between non-blocking transactions depends onordering required for keeping memory coherent. However, in an example,read responses are arbitrated in any order, since there is no hazardbetween read responses.

Stages P0 (420) through P3 (426) are non-stalling and non-blocking. Thenon-stalling nature means that a transaction does not stall in thesepipeline stages. In an example, transactions take either 1 or 2 cycles,has guaranteed slots in the following pipeline stage. The non-blockingnature relies on the fact that the arbitration before P0 420 hasguaranteed that a FIFO entry is available for the transaction enteringP0 420, and for any secondary transactions that it may generate.

The stage P0 420 generally performs a credit management function, inwhich credits are “consumed” by certain transactions based on thetransaction type. These consumed credits are released later in thepipeline 400. The concept of credits is one exemplary approach toensuring that transactions are allowed to advance only when the have amemory element to land in a later pipe stage, which ensures thenon-blocking characteristics of the pipeline. However, other examples donot necessarily rely on credits, but employ other methods to ensure thattransactions are allowed to advance only when there is sufficientpipeline space to allow the transaction to proceed through the pipelinestage(s) that are non-blocking.

The stage P0 420 along with stages P1 422 and P2 424 perform variouscache and SRAM functionality, such as setting up reads to variouscaches, performing ECC detection and/or correction for various caches,and determining cache hits and misses. The stage P3 426 performsadditional cache hit and miss control, and also releases credits forcertain transaction types.

Examples of this disclosure are directed to dynamic arbitration ofvarious transactions in the pipeline stage P4 428 and the cache missarbitration and send stage, which is described in further detail below.Referring to FIG. 5, a system 500 is shown that includes an exemplary P4stage 428 from one of the pipelines 400. Although not shown forsimplicity, it should be appreciated that the other pipelines contain asimilar P4 stage that functions in a manner similar to the P4 stage 428described below. As shown, the P4 stage 428 includes FIFOs for varioustransaction types. For example, the P4 stage 428 includes a FIFO fortype 0 blocking transitions 502, a FIFO for type 1 non-blockingtransactions 504, and a FIFO for type 2 non-blocking transactions 506.The specific transaction types are explained in further detail below.The output of each FIFO 502, 504, 506 is input to a multiplexer 508,which is controlled by a dynamic arbitration state machine 510, whichwill also be explained in further detail below. The output of each P4stage 428 is made available to various FIFOs 512 of the cache missarbitration and send stage, which is a single stage where transactionsfrom all pipes are arbitrated, multiplexed and sent out from the L2cache subsystem 306, for example to the L3 cache subsystem 308.

The FIFO 502 receives type 0 transactions from the previous pipe stages,which include all blocking read and write transactions. The FIFO 504receives type 1 transactions from the previous pipe stages, whichinclude non-blocking victims or snoop responses from L1D 204. The FIFO506 receives type 2 transactions from the previous pipe stages, whichinclude non-blocking L2 victims or snoop responses that hit the L2 cache324.

As explained, the cache miss arbitration and send stage is a stage thathandles transactions from all pipes. Transactions from any pipe that areintended for the L3 cache subsystem 308 are arbitrated in this stage. Inan example, this arbitration is isolated and independent from thetransactions from every pipe that are intended for the L1 cachesubsystem 304. The cache miss arbitration and send stage evaluates thetype and number of credits required to send a particular transaction outto the L3 cache subsystem 308 endpoint based on the transaction type,and arbitrates one transaction from the pipes that can go out (e.g.,using arbitration logic 514 to control entry into the various FIFOs512).

In one example of the cache miss arbitration and send stage, the outputFIFOs 512 include different structures having variable, configurabledepths. In this example, the global FIFO can accept blocking andnon-blocking transactions. The blocking FIFO can accept cache allocatesand blocking read and write transactions. A blocking transaction ispushed into the blocking FIFO when the global FIFO is full. Thenon-blocking FIFO can accept snoop responses and L1 cache subsystem 304and L2 cache subsystem 306 victims. A non-blocking transaction is pushedinto the non-blocking FIFO when the global FIFO is full. Transactionsare released from the FIFOs 512, for example, based on interactions withthe L3 cache subsystem 308 that indicate whether and/or how muchtransaction processing bandwidth is available in the L3 cache subsystem308, and for what types of transactions (e.g., a credit-based scheme).The read response FIFO is used for DMA read responses, which arereleased to the L3 cache subsystem 308 on a DMA thread.

In an example, a FIFO full signal is sent from the output FIFOs 512 tothe P4 stage 428. In one example, the FIFO full signal actually includesa separate signal for each of the FIFOs 512. These separate signals areasserted when the corresponding FIFO 512 is full, and de-asserted whenthe corresponding FIFO 512 is not full. As will be explained furtherbelow, this insight into the status of the FIFOs 512 in the next stageallows the dynamic arbitration state machine 510 of the P4 stage 428 tomore efficiently arbitrate among various transactions (e.g., type 0,type 1, type 2).

In particular, the FIFO full signal indicates that the FIFO(s) 512 thata transaction (e.g., being considered by the dynamic arbitration statemachine 510) is trying to advance to has no empty slots. The statemachine 510 monitors the specific signal(s) of the FIFO full signal forthe FIFO(s) 512 to which it could advance a transaction. In exampleswhere a transaction comprises two data phases, explained further below,the FIFO full signal indicates the availability of two data slots in theFIFO(s) 512.

In accordance with examples of this disclosure, the dynamic arbitrationstate machine 510 of the P4 stage 428 monitors the transactions from theprevious stage P3 426, as well as the availability of the FIFOs 512(e.g., through the FIFO full signals). As explained, the previous stageP3 426 can send transactions of type 2, type 1, or type 0 to the P4stage 428. Type 2 transactions have the highest priority, while type 0transactions have the lowest priority, based on the blocking andnon-blocking rules explained above.

FIG. 6 shows a flow chart 600 of the operation of the dynamicarbitration state machine 510. The chart 600 (e.g., the state machine510) begins in the state 602 in which the state machine 510 monitorstransactions from stage P3 426. For example, the FIFOs 502, 504, 506 areinitially empty, and thus when a transaction from stage P3 426 isreceived, the state machine 510 is aware of the transaction's presencein one of the FIFOs 502, 504, 506. When a transaction is received in oneof the FIFOs 502, 504, 506, the state machine 510 proceeds to block 604to determine whether the transaction is of a highest priority level(e.g., type 2 in the example above, in the FIFO 506). If a type 2transaction is available, the state machine 510 proceeds to block 606.

In the example of FIG. 6, it is assumed that transactions are processedas two data phases (DP). For example, the unit of coherence for a cacheline is 128 bytes, while a physical bus width is only 64 bytes (e.g.,the data phase), and thus transactions are split into first and seconddata phases. In another example where transactions are single DPtransactions, the state machine 510 is simplified by eliminating theneed to send a second DP before again monitoring for new transactionsfrom the FIFOs 502, 504, 506.

Since it is assumed that transactions are have two DPs, the statemachine 510 proceeds to block 606 where the first DP and command is sentto be arbitrated for entry into the FIFOs 512. When the cache missarbitration stage accepts the first DP, it transmits an ACK signal tothe state machine 510. The state machine 510 waits to receive the ACKbefore proceeding to block 608 and sending the second DP to bearbitrated for entry into the FIFOs 512. In this example, the ACKarrives the cycle after the first DP and command is sent by the P4 stage428 to the cache miss arbitration stage.

After the second DP is sent, the state machine 510 proceeds to block 610to determine whether the transaction is of a highest priority level(e.g., type 2). If a type 2 transaction is available in the FIFO 506,the state machine 510 returns to block 606 and proceeds as explainedabove. As a result, as long as a type 2 transaction is available in theFIFO 506, the state machine 510 continues to give highest priority tothose transactions.

However, if a type 2 transaction is not present in the FIFO 506 (eitheras determined in block 604 or block 610), the state machine proceeds toblock 612 to determine whether a transaction is available in the FIFO504 (e.g., is a type 1 transaction). If a type 1 transaction isavailable in the FIFO 504, the state machine 510 continues to block 614.As above, it is assumed that transactions are have two DPs, and so thestate machine proceeds in block 614 to send the first DP and command tobe arbitrated for entry into the FIFOs 512.

Unlike when processing a type 2 transaction having the highest priority,while no ACK is yet received, the state machine 510 proceeds to block616 to check the FIFO full signal. As long as the FIFO full signal isnot asserted (e.g., for the FIFO(s) 512 pertaining to the type 1transaction), the state machine 510 returns to block 614 to continue towait for an ACK. However, if the FIFO full signal is asserted, thenthere is no room in the FIFO(s) 512 pertaining to the type 1transaction, and the state machine 510 continues to block 618 todetermine whether a type 2 transaction is available in the FIFO 506. Asabove, if a lower-priority transaction cannot be completed (e.g., due toFIFOs 512 being full), the state machine 510 prioritizes the highestpriority, type 2 transactions if available in the FIFO 506. If a type 2transaction is available, the state machine 510 returns to block 606 toprocess the type 2 transaction as described above. If, in block 618, itis determined that a type 2 transaction is not available, the statemachine 510 returns to block 616 to determine whether the FIFO fullsignal is still asserted.

The above-described loop between blocks 616, 614, and 618 continuesuntil an ACK is received, at which point the state machine 510 proceedsfrom block 614 to block 620 and sends the second DP to be arbitrated forentry into the FIFOs 512. Once the second DP has been sent, the statemachine 510 waits for an ACK in block 620 and proceeds back to block 602to monitor the transactions in FIFOs 502, 504, 506.

Referring back to block 612, if a type 1 transaction is not available inthe FIFO 504, then a transaction of type 0 is available in the FIFO 502and the state machine 510 continues to block 624. As above, it isassumed that transactions are have two DPs, and so the state machineproceeds in block 624 to send the first DP and command to be arbitratedfor entry into the FIFOs 512.

As above with processing a type 1 transaction, while no ACK is yetreceived, the state machine 510 proceeds to block 626 to check the FIFOfull signal. As long as the FIFO full signal is not asserted (e.g., forthe FIFO(s) 512 pertaining to the type 0 transaction), the state machine510 returns to block 624 to continue to wait for an ACK. However, if theFIFO full signal is asserted, then there is no room in the FIFO(s) 512pertaining to the type 0 transaction, and the state machine 510continues to block 628 to determine whether a type 2 transaction isavailable in the FIFO 506 or a type 1 transaction is available in theFIFO 504. As above, if a lower-priority transaction cannot be completed(e.g., due to FIFOs 512 being full), the state machine 510 prioritizesthe higher priority, type 2 transactions (if available in the FIFO 506)and type 1 transactions (if available in the FIFO 504). If a type 2 ortype 1 transaction is available, the state machine 510 returns to block604 to determine whether a type 2 or type 1 is available, and the statemachine 510 operates as described above. If, in block 628, it isdetermined that a type 2 or type 1 transaction is not available, thestate machine 510 returns to block 626 to determine whether the FIFOfull signal is still asserted.

The above-described loop between blocks 626, 624, and 628 continuesuntil an ACK is received, at which point the state machine 510 proceedsfrom block 624 to block 600 and sends the second DP to be arbitrated forentry into the FIFOs 512. Once the second DP has been sent, the statemachine 510 waits for an ACK in block 630 and proceeds back to block 602to monitor the transactions in FIFOs 502, 504, 506.

Thus, the dynamic arbitration state machine 510 prioritizes ahigher-priority transaction frequently, to ensure that the inability ofa lower-priority transaction to proceed to the next stage does notinterfere with the processing of such higher-priority transactions.

Additionally, by checking the FIFO full signals during processing ofvarious transactions, the state machine 510 remains aware of whether aparticular transaction can proceed from the stage P4 428. For example, atransaction cannot proceed from the P4 stage 428 to the cache missarbitration and send stage if FIFO full signal is asserted. The FIFOfull signal being low indicates that the transaction being operated onby the dynamic arbitration state machine 510 will eventually be able toenter one of the FIFOs 512 (although in some cases it may be stalledtemporarily). For example, if another pipeline's P4 stage is able toadvance a transaction to the cache miss arbitration and send stage, thena FIFO 512 may become full, causing the FIFO full signal to be asserted.However, if the FIFO 512 has an available slot, the FIFO full signalremains de-asserted. Finally, if the state machine 510 is stalled, forexample because the FIFO full signal is asserted, then the transactioncannot advance. If a transaction with a higher priority arrives, thestate machine 510 switches to process the higher-priority transaction.The transaction that was being processed may be temporarily held, orparked (e.g., in a memory structure, which in some examples is differentthan the FIFOs 502, 504, 506, 512), until the state machine 510 hasprocessed the higher-priority transaction, at which point the statemachine 510 returns to process the lower priority transaction.

In the example of FIG. 6, it was assumed that transactions are processedas two data phases (DP), due to the data phase size being smaller thanthe transaction size. However, in other examples, transactions areprocessed as a single data phase, and thus blocks 608, 620, and 630 areremoved from the state machine in FIG. 6. In another example, ratherthan having high, medium, and low priority transactions (e.g., type 2,type 1, and type 0 transactions, respectively), transactions areclassified as either high priority or low priority. In this example,blocks 612 and 624-630 are removed from the state machine in FIG. 6. Inyet another example, rather than having multiple input transactionbuffers 502, 504, 506, these buffers are be condensed to fewer buffers,including in some examples a single buffer. Similarly, rather thanhaving multiple output buffers 512, these buffers are condensed to fewerbuffers, including in some examples a single buffer.

In examples of the present disclosure, global cache operations arepipelined to take advantage of the banked configuration of the L2 cachesubsystem 306, explained above. A global cache operation is atransaction that operates on more than one cache line. In addition, theL2 controller 320 manages global cache operations on the L2 cachesubsystem 306 to avoid encountering any blocking conditions during theglobal cache operation.

As explained, the L2 cache subsystem 306 includes multiple banks in someexamples (e.g., banks 224 a-224 d shown above in FIG. 2). In certainexamples, the number of banks is configurable. Each bank has anindependent pipeline 400 associated therewith. Thus, the L2 controller320 is configured to facilitate up to four transactions (in the exampleof FIG. 2) to the L2 cache 324 in parallel (e.g., one transaction perbank). In accordance with examples of this disclosure, this enables theL2 controller 320 to facilitate global coherence operations on the banksof the L2 cache 324 at the same time.

FIG. 7 shows a flow chart of a method 700 for stalling a pipeline of theL2 cache subsystem 306 (e.g., pipeline 400, described above) to performa global cache operation in accordance with various examples of thisdisclosure. The method 700 begins in block 702, which is the start ofthe global operation state machine. In block 702, the L2 controller 320receives a request to perform a global operation on the L2 cache 324. Insome examples, the request is in the form of a program (e.g., executedby the CPU core 302) asserting a field in a control register, such asthe ECR 228.

Various global cache operations are able to be requested of the L2controller 320. In one example, the global cache operation is aninvalidate operation, which invalidates each cache line in the L2 cache324. In another example, the global operation is a writeback invalidateoperation, in which dirty cache lines (e.g., having a coherence state ofmodified) in the L2 cache 324 are written back to their endpoint andsubsequently invalidated. In yet another example, the global operationis a writeback operation, in which dirty cache lines in the L2 cache 324are written back to their endpoint. The written back, dirty cache linesin the L2 cache 324 then have their coherence state updated to a sharedcache coherence state. In some of these examples, the global operationcomprises querying the cache coherence state of each line in the L2cache 324 and updating the cache coherence state of each line in the L2cache 324. For example, if the global operation is the writebackoperation, after modified cache lines in the L2 cache 324 are writtenback to their endpoint, the L2 controller 320 queries the coherencestate for the lines in the L2 cache 324 and updates the coherence statefor modified cache lines to be shared.

Regardless of the type of global cache operation to be performed, forexample as indicated in the request to the L2 controller 320 (e.g.,based on an asserted field of a control register, such as ECR 228), themethod 700 continues to block 704 in which the L2 controller 320enforces a blocking soft stall. In the blocking soft stall phase, the L2controller 320 stalls all new blocking transactions from entering thepipeline, while permitting non-blocking transactions including responsetransactions, non-blocking snoop, and victim transactions to be acceptedinto the pipeline and arbitrated.

In an example, multiple cycles are needed for the L2 controller 320 toflush its pipeline in the blocking soft stall phase 704. Thus, themethod 700 continues in block 706 to determine whether all blockingtransactions have been flushed from the pipeline. In response to anindication that the pipeline does not contain any more blockingtransactions, the method 700 continues to block 708 in which the L2controller 320 enforces a non-blocking soft stall. In the non-blockingsoft stall phase, the L2 controller 320 stalls new snoop transactionsfrom entering the pipeline, while permitting new response transactionsand victim transactions to enter the pipeline. The non-blocking softstall phase thus prevents new snoops from being initiated to the L1controller 310 for lines previous cached in the L1 cache 314.

The method 700 continues in block 710 to determine whether all snooptransactions have been flushed from the pipeline. In response to anindication that the pipeline does not contain any more pending snooptransactions, the method 700 continues to block 712 in which the L2controller 320 enforces a hard stall. In the hard stall phase, the L2controller 320 prevents all new transactions from entering the pipeline,including response transactions.

In some examples, the L2 controller 320 de-asserts a ready signal duringthe soft and hard stall phases. De-asserting the ready signal indicatesto the CPU core 302 not to send the L1 controller 310 additionalrequests for a global coherence operation or a cache size change. Thus,the L2 controller 320 is able to complete the pending global coherenceoperation while guaranteeing that additional global coherence operationswill not be issued by the CPU core 302. The ready signal remainsde-asserted until the global operation is completed.

The method continues in block 714 to determine whether all transactionshave been flushed from the pipeline. In response to the L2 controller320 determining that the pipeline does not contain any more pendingtransactions, the method 700 continues to block 716. The method 700steps of 702 through 714 are performed by the L2 controller 320, forexample, on each pipeline independently (e.g., as a state machineimplemented for each pipeline) and in parallel. However, in block 716,the L2 controller 320 waits for confirmation from all pipelines thatthey have flushed all pending transactions (e.g., that all pipelineshave proceeded to block 716). Once confirmation is received that allpipelines have flushed all pending transactions, the method 700continues to block 718 where the global operation is performed. In anexample, the global operation also proceeds independently, in parallelon each of the pipelines to the banked L2 cache 324. An applicationexecuting on the CPU core 302 that requested the global operation beperformed (e.g., by asserting a field in a control register such as ECR228) is also configured to poll the same field, which the L2 controller320 is configured to de-assert upon completion of the global operation.

By stalling its pipelines in a phased manner as described above, the L2controller 320 first avoids continuing to process transactions thatcould change the state of the L2 cache 324 (e.g., a read request thatcauses a change to the cache coherence state of a cache line). While theL2 cache 324 will not receive any more transactions that could changeits state, the L2 controller 320 continues to process certaintransactions that resulted from a transaction that occurred before theglobal operation was requested. For example, if the L1 controller 310issued a victim to the L2 controller 320 as a result of a read beforethe global operation, the L2 controller 320 does not necessarily knowwhat read request caused the victim from the L2 controller 310, and thuscontinues to process such victims (and snoop responses) as a saferapproach. The L2 controller 320 does not continue to send out newtransactions, because this could lead to a loop condition. Snooptransactions before the global operation continue to be processed (e.g.,in block 710) and once those snoop transactions are processed, the L2controller 320 has successfully stopped new transactions from beingprocessed, and processed those transactions already in progress tocompletion. The parallel performance of a global operation thus enabledby the L2 controller 320 improves performance from the parallel natureof the banked L2 cache 324 and the parallel implementation of globaloperations.

A write request received from the CPU core 302 that can be cached in theL2 cache 324, but that misses the L2 cache 324, can be“write-allocated.” Examples of this disclosure relate to certainimprovements enabled by the L2 controller 320 and associated structuresof the L2 cache subsystem 306 for such write allocate transactions.

In an example, the L2 cache subsystem 306 includes memory storageelements (e.g., buffers) that are used to service write allocatetransactions. These are referred to as register files herein, althoughthis disclosure should not be construed to be limited to a specific typeof memory element. FIG. 8b , discussed further below, shows an exampleof register files used to service write allocate transactions.

When the L2 controller 320 determines to perform a write allocate (e.g.,when a write request misses the L2 cache 324), the L2 controller 320 isconfigured to generate a read request to the address to be written tointo the L2 cache subsystem 306. That is, rather than forward the writerequest to the L3 controller 309 or DDR 110, the L2 controller 320 isconfigured to bring the data to be written to into the L2 cachesubsystem 306 to ultimately be stored in the L2 cache 324.

The write request received by the L2 controller 320 includes write datain a data field, and in some cases also includes an enable field, whichspecifies valid portions of the data field (e.g., those containing validwrite data). The enable field is described further below. Regardless, insome cases, the L2 controller 320 allocates space in a register file forthe data associated with the write request (e.g., the data field andpossibly the enable field). Additionally, the L2 controller 320allocates space in the register file for the read response that isexpected to result from the read request that the L2 controller 320issued as a result of the write allocate. When the read response isreceived, the L2 controller 320 writes the read response data to a linein the L2 cache 324 and then writes the write data to the same line inthe L2 cache 324, completing the initial write request. However, thisapproach requires more storage in the register file and increases thenumber of transactions that are carried out to finally implement thewrite request.

In examples of this disclosure, the L2 controller 320 is configured toreserve an entry in a register file for read data returned in responseto the read request that resulted from the write allocate transaction.The L2 controller 320 updates a data field of the reserved entry withthe write data (e.g., the data field of the initial write request) andthe L2 controller 320 updates an enable field of the reserved entrybased on the write data. Then, when the read response is returned, theL2 controller 320 is configured to merge the returned read data into thedata field of the reserved entry. The reserved entry is then written tothe L2 cache 324. This reduces the space required in the register fileto service such a write allocate transaction. Additionally, transactionsto the L2 cache 324 are reduced since the merging occurs in the registerfile of the L2 cache subsystem 306.

FIG. 8a shows an example 800 of the above functionality, which enablesthe L2 controller 320 to improve cache allocation, particularly inresponse to a write request. The example 800 includes an initialsnapshot of an entry in a register file after a write request has beenreceived by the L2 controller 320 that misses the L2 cache 324. In thisexample 800, the write request is for address A. The write data includesx0A in a first portion 802 of the data field and x0B in a second portion804 of the data field. In this example, the enable field comprises onebit per byte of data in the data field, which is asserted when thecorresponding data field portion is valid. Thus, the enable field forthe first and second portions 802, 804 is asserted. Conversely, theenable field for third and fourth portions 806, 808 is de-asserted, andthus the data fields in the third and fourth portions 806, 808 areirrelevant as invalid write data.

The example 800 also includes a later snapshot of the entry in theregister file after a read response (e.g., a response to the readrequest that the write allocate transaction caused) has been received bythe L2 controller 320. In this example 800, the data contained ataddress A is xCDEF9876. As explained above, the L2 controller 320 isconfigured to merge the write data with the read response in the entry.In particular, the valid write data (indicated by an assertedcorresponding enable field) overwrites the read response data inportions 810 and 812, while the read response data that is notoverwritten (due to a de-asserted corresponding enable field) remains inthe entry in portions 814, 816. In particular, when a sub-field orportion of the enable field is asserted (e.g., portions 802 and 804),merging the write data with the read response in the entry includesdiscarding the read data. Similarly, when a sub-field or portion of theenable field is de-asserted (e.g., portions 806 and 808), merging thewrite data with the read response includes replacing the portion of thedata field (e.g., a byte in the example 800) associated with thede-asserted sub-field with a corresponding portion of the read data(e.g., a byte in the example 800). Although not depicted, the readresponse can also be returned as mutually exclusive fragments, and thusmerging is handled in a similar way.

FIG. 8b shows example register files 850 containing entries as describedabove. The example register files 850 are included in the L2 cachesubsystem 306. In particular, the example 850 depicts the register filesas schematically separate blocks including a write-allocate address FIFO852, a write-allocate data FIFO 854, and a write-allocate enable FIFO856. Although these are labeled as FIFOs, the structure of the registerfiles is not necessarily a first-in, first-out structure in allexamples. In accordance with the examples of this disclosure, write datais written to an entry in each of the FIFOs 852, 854, 856 when the L2controller 320 generates the read request to the next level cache (e.g.,the L3 cache subsystem 308). In this example, the write data includesthe write-allocate address, which is written to the write-allocateaddress FIFO 852. The write data also includes the actual write dataitself, which is written to the write-allocate data FIFO 854. Finally,the write data includes the enable data (e.g., one bit per byte of writedata) that specifies whether a write data field is valid, which iswritten to the write-allocate enable FIFO 856. Upon the return of datafrom the address in the form of a read response (e.g., from the L3 cachesubsystem 308), the read data is merged with the write data in the entryof the write-allocate data FIFO 854, for example based on thecorresponding enable data in the write-allocate enable FIFO 856 asexplained above with respect to FIG. 8 a.

FIG. 9 shows a flow chart of a method 900 for improving cache allocationin response to a write request. The method 900 begins in block 902 withthe L2 controller 320 receiving a write request for an address that isnot allocated as a cache line in the L2 cache 324. The write requestincludes write data.

The method 900 continues in block 904 with the L2 controller 320generating a read request for the address of the write request. Themethod 900 then continues in block 906 with reserving an entry in aregister file for read data returned in response to the generated readrequest.

The method 900 continues further in blocks 908 and 910 with the L2controller 320 updating a data field of the entry in the register filewith the write data, and updating an enable field of the entryassociated with the write data, respectively. As explained above, theenable field indicates the validity of a corresponding portion of thewrite data, and in the example of FIG. 8a comprises one bit per byte ofwrite data. Finally, the method 900 concludes in block 912 with the L2controller 320 receiving the read data and merging the read data intothe data field of the entry, for example as described above with respectto FIG. 8 a.

These improvements to write allocates in the L2 cache subsystem 306reduce the space required in the register file to service such a writeallocate transaction. Additionally, transactions to the L2 cache 324 arereduced because the merging occurs in the register file of the L2 cachesubsystem 306.

The selection of a cache replacement algorithm can impact theperformance of a cache subsystem, such as the L2 cache subsystem 306explained above.

In an example, the L2 cache 324 is a read and write allocatable 8-waycache. The allocation of a cache line in the L2 cache 324 depends onvarious page attributes, cache mode settings, and the like. On detectingthat a line is not present in the L2 cache 324 (e.g., a cache miss), theL2 controller 320 decides to allocate a line. For the sake of brevity,it is assumed that the L2 controller 320 is permitted to allocate theline upon the cache miss. The following examples explain how the L2controller 320 allocates the line.

In some examples, the L2 controller 320 is configured to pipelineallocations to the L2 cache 324. As a result, the L2 controller 320could end up in a situation where multiple cache line allocations aresent to the same way. Because response data can come out of order, thiscan cause data corruption, if multiple lines are allocated to the sameway in the L2 cache 324. On the other hand, if multiple cache lines areto the same set, it is advantageous to avoid constraining the L2controller 320 by the number of ways (8) to send the allocations out.

As explained above, each line in the L2 cache 324 comprises a coherencestate (e.g., a MESI state, requiring 2 bits). Additionally, a secure ornon-secure status (e.g., requiring 1 bit) of the line is tracked by theL2 controller 320. However, the security state of a line having acoherence state of invalid is not pertinent, and thus an additionalcache line state is able to be tracked by the L2 controller 320 withoutrequiring any additional replacement bit overhead. It is advantageous toreduce the replacement bit overhead employed by a particular replacementalgorithm.

As one example, the following are possible coherence states for a linein the L2 cache 324:

-   -   “000”: INVALID—Way is empty and available for allocation    -   “001”: PENDING—Way is empty, but has been marked for allocation    -   “010”: SHARED__NON_SECURE—The line allocated to this way is in        the Shared MESI state and is a non-secure line    -   “011”: SHARED__SECURE—The line allocated to this way is in the        Shared MESI state and is a secure line    -   “100”: EXCLUSIVE__NON_SECURE—The line allocated to this way is        in the Exclusive MESI state and is a non-secure line    -   “101”: EXCLUSIVE__SECURE—The line allocated to this way is in        the Exclusive MESI state and is a secure line    -   “110”: MODIFIED__NON_SECURE—The line allocated to this way is in        the Modified MESI state and is a non-secure line    -   “111”: MODIFIED__SECURE—The line allocated to this way is in the        Modified MESI state and is a secure line

As explained above, this enables Bit_0 of this status field to be usedfor both indicating that the line is pending, and as a secure bit if theline has already been allocated. This reduces the storage needed forholding this status information. For ease of explanation, pending isalso considered a cache coherence state for purposes of describing thecache replacement polices below.

As used herein, pending refers to a situation where the L2 controller320 has decided to allocate the line and has made a decision as to whichway it will be allocated. This way is essentially locked to otherallocates and stores the response data upon arrival. In accordance withexamples of this disclosure, the L2 controller 320 leverages the pendingbit to determine which of the ways are available for new allocations,which improves performance over a purely random cache replacementpolicy.

In accordance with examples of this disclosure, the L2 controller 320employs a pseudo-random replacement policy. In the event that there isat least one way in a set that is available (e.g., having a cachecoherence state of invalid), the L2 controller 320 is configured to pickthat way for allocation. However, if all ways in the set have a cachecoherence state of pending, the L2 controller 320 cannot select a wayfor allocation. Rather than stalling the transaction, the L2 controller320 is configured to convert the transaction to a non-allocatable accessand forwards the transaction to the endpoint (e.g., the L3 cachesubsystem 308). As a result, the L2 controller 320 continues to pipelineout accesses without an unnecessary stall of transactions.

Finally, if there are no empty (e.g., invalid) ways in the set, then theL2 controller 320 utilizes a random number generator to identify a wayin the set. FIG. 10 shows an example 1000 of a mask-based way selectionusing the random number generator. In particular, the set includes eightways as shown in block 1002. Block 1004 demonstrates that ways 0, 1, 4,and 7 have pending cache coherence states. Mask logic 1006 is applied tothe blocks 1002 and 1004 to create a masked subset that includes theways of the set that are not pending, which are ways 2, 3, 5, and 6 asshown in block 1008. If all ways are pending in block 1010, or themasked subset in block 1008 is empty, then the L2 controller 320converts the transaction to a non-allocatable access (e.g., to the L3controller 309) in block 1012, and as described above. However, if notall ways are pending in block 1014, then the L2 controller 320 appliesthe random number generator to select from the eligible ways in block1008. In block 1016, the way selected in block 1014 has its cache stateupdated to pending and the L2 controller 320 sends an allocate requestto, for example, the L3 controller 309.

FIG. 11 shows a flow chart of an alternate method 1100 of using therandom number generator for way selection. The method 1100 begins inblock 1102 with the L2 controller 320 receiving a first request toallocate a line in the L2 cache 324, which is an N-way set associatedcache as explained. In response to a cache coherence state of a wayindicating that a cache line stored in the way is invalid, the method1100 continues in block 1104 with the L2 controller 320 allocating theway for the first request. This is similar to the behavior describedabove.

However, in response to no ways in the set having a cache coherencestate indicating that the cache line stored in the way is invalid, themethod 1100 continues in block 1106 with the L2 controller 320 using therandom number generator to randomly select one of the ways in the set.In the method 1100, the random number generator is utilized withoutfirst masking pending ways, which reduces processing requirements. Inresponse to a cache coherence state of the randomly selected wayindicating that another request is not pending for the selected way(e.g., the randomly selected way has a coherence state other thanpending), the method 1100 continues in block 1108 with the L2 controllerallocating the selected way for the first request.

In the event that the randomly selected way in the method 1100 has acoherence state of pending, the L2 controller 320 can choose to servicethe first request without allocating a line in the L2 cache 324, forexample by converting the first request to a non-allocating request andsending the non-allocating request to a memory endpoint identified bythe first request. In other examples, upon the randomly selected wayhaving a coherence state of pending, the L2 controller 320 is configuredto randomly select another of the ways in the set. In some examples, theL2 controller 320 is configured to randomly re-select in this manneruntil the cache coherence state of the selected way does not indicatethat another request is pending for the selected way. In other examples,the L2 controller 320 is configured to randomly re-select in this manneruntil a threshold number of random selections have been performed.

Regardless of the particular approach to random way selection employed,as described above, in the situation that the L2 controller 320 does notallocate the line (e.g., converts the request to a non-allocatingrequest), performance is enhanced by not stalling the CPU core 302, andthe L2 controller 320 continues sending accesses out to, for example,the L3 controller 309.

As explained above, the L3 cache subsystem 308 includes L3 SRAM, and insome examples of this disclosure the L3 SRAM address region existsoutside of the L2 cache subsystem 306 and the CPU core 302 addressspace. Depending on performance requirements of various applications,the L3 SRAM address region is considered as shared L2 or L3 memory. Oneway to implement the L3 SRAM address region as shared L2 or L3 memory isto disable the ability of the L2 cache subsystem 306 to cache anyaddress that mapped to the L3 SRAM address region. However, if anapplication does not need to use the L3 SRAM as shared L2 or L3 memory(e.g., to enable the L2 cache subsystem 306 to cache addresses in the L3SRAM address region), the physical L3 SRAM region is mapped (e.g.,through the MMU described above) to an external, virtual address. Thismapping requires additional programming (e.g., of the MMU), and the L2controller 320 has to manage different addresses mapping to the samephysical L3 SRAM address region, which adds complexity for thoseapplications that enable the L2 cache subsystem 306 to cache addressesin the L3 SRAM address region.

In accordance with examples of this disclosure, the L2 cache subsystem306 includes a caching configuration register (e.g., a register or afield of ECR 228) that allows configurable control of whether the L2cache subsystem 306 is able to cache addresses in the L3 SRAM addressregion. In some examples, the L3 SRAM includes multiple address regions,and the caching configuration register establishes whether each addressregion is cacheable or non-cacheable by the L2 cache subsystem 306. Forsimplicity, it is assumed that the L3 SRAM is a single address region,and thus the cacheability of the L3 SRAM address region is controllableby, for example, a single bit in the caching configuration register.

For example, in response to the caching configuration register having afirst (e.g., de-asserted) value, the L2 controller 320 is configured tooperate in a non-caching mode, in which the L2 controller 320 providesrequests to the L3 cache subsystem 308 but does not cache any datareturned by the request. However, in response to the cachingconfiguration register having a second (e.g., asserted) value, the L2controller 320 is configured to operate in a caching mode, in which theL2 controller 320 provides requests to the L3 cache subsystem 308 andcaches any data returned by the request, for example in the L2 cache324.

As a result, when the L2 controller 320 operates in the non-cachingmode, the L3 SRAM address region can be shared among multiple CPU cores(e.g., CPU cores 102 a-102 n), without any cache-related performancepenalties, such as increased transaction volume to maintain cachecoherence (e.g., victim transactions). However, the L2 controller 320also has the flexibility to cache the L3 SRAM address region when, forexample, a particular application benefits from such behavior (e.g.,data stored in L3 SRAM is infrequently shared among CPU cores).

In an example, when the L2 controller 320 transitions from thenon-caching mode to the caching mode (e.g., the caching configurationregister or field thereof is asserted), the L2 controller 320 typicallycan begin caching addresses from the L3 SRAM address region withoutadditional actions being taken. For example, because the L2 controller320 had not previously been caching these addresses, there are noimpediments to the L2 controller 320 simply beginning operation in thecaching mode.

However, when it is determined (e.g., by the CPU core 302) to transitionthe L2 controller 320 from the caching mode to the non-caching mode(e.g., the caching configuration register or field thereof isde-asserted), additional steps may be performed before the L2 controller320 transitions to the non-caching mode. For example, steps are taken toevict from the L2 cache 324 any lines that were cached from the L3 SRAMaddress region.

In this example, traffic from the CPU core 302 for addresses that map tothe L3 address region is ceased. For example, the CPU core 302 (or anapplication executing thereon) that requested the L2 controller 320 totransition from caching mode to non-caching mode (e.g., throughde-assertion of the configuration register) ceases to send requests tothe L2 controller 320 directed to addresses in the L3 SRAM. At the sametime, the CPU core 302 can continue to send requests to the L2 cachesubsystem 306 directed to addresses other than in the L3 SRAM addressregion.

Then, for example in response to the de-assertion of the cachingconfiguration register, the L2 controller 320 is configured to evictcache lines in its L2 cache 324 that correspond to the L3 SRAM addressregion. The L2 controller 320 can evict all cache lines in its L2 cache320 or only those that correspond to the L3 SRAM address region. In oneexample, the L2 controller 320 invalidates each line in the L2 cache 324that corresponds to the L3 SRAM address region. In another example, theL2 controller 320 writes back each line in the L2 cache 324 thatcorresponds to the L3 SRAM address region. In yet another example, theL2 controller 320 performs a writeback invalidate of each line in the L2cache 324 that corresponds to the L3 SRAM address region. Examples ofthis disclosure are not necessarily restricted to a specific form of theeviction of lines from the L2 cache 324 corresponding to the L3 SRAMaddress region.

Continuing the writeback invalidate example, the L2 controller 320performs the writeback invalidate of either its entire L2 cache 324 orthe portions of the L2 cache 324 that correspond to the L3 SRAM addressregion. In one example, the L2 controller 320 performs a writebackinvalidate operation, while in another example the streaming engine 205is used to perform a block writeback (e.g., of the addresses in the L2cache 324 that correspond to the L3 SRAM address region). The L2controller 320 indicates the completion of the writeback invalidate, forexample by asserting a signal to the CPU core 302 or changing awriteback invalidate register value that is polled by the CPU core 302.Once the CPU core 302 receives the indication that the writebackinvalidate is complete, the CPU core 302 de-asserts the cachingconfiguration register to disable caching of the L3 SRAM address regionby the L2 cache subsystem 306. The CPU core 302 is then able to resumesending requests to the L2 cache subsystem 306 for addresses in the L3SRAM address region, which will not be cached by the L2 controller 320.

FIG. 12 shows a flow chart of a method 1200 for operating a cachecontroller (e.g., L2 controller 320) in a caching or a non-caching mode,in accordance with various examples. The method 1200 begins in block1202 with the L2 controller 320 receiving a request directed to anaddress in the L3 SRAM address region. In block 1204, it is determinedwhether the caching configuration register has a first value (e.g., isde-asserted) or a second value (e.g., is asserted). If the cachingconfiguration register is de-asserted, the method 1200 continues toblock 1206 in which the L2 controller 320 operates in the non-cachingmode by providing the request to the L3 SRAM (e.g., via the L3controller 309). The method 1200 then continues to block 1208 in whichthe L2 controller 320 does not cache data returned by the request in itsL2 cache 324.

Returning to block 1204, if the caching configuration register isasserted, the method 1200 continues to block 1210 in which the L2controller 320 operates in the caching mode by providing the request tothe L3 SRAM (e.g., via the L3 controller 309). The method 1200 thencontinues to block 1212 in which the L2 controller 320 caches datareturned by the request in its L2 cache 324.

Examples of the present disclosure relate to operating the L2 controller320 to permit accesses to the L2 SRAM 322 in both aliased and un-aliasedmodes. In some cases, prior versions of processors utilized anon-programmable, static implementation in hardware (e.g., usingmultiplexers) to operate in an aliased mode. In this approach, memorywas statically structured as three separate memories that could not bemerged into one common memory map. Additionally, multiplexing applied toall transactions and requestors, and thus it was not possible to operatein an un-aliased mode. The examples described herein enable legacyapplications to continue to utilize aliased mode as needed whenaccessing the L2 SRAM 322, but also does not restrict the L2 SRAM 322 tostrictly aliased accesses, which increases the functionality andflexibility of the L2 cache subsystem 306 more generally.

FIG. 13 shows an example and block diagram 1300 of un-aliased andaliased modes of operation (e.g., of the L2 controller 320 interactingwith the L2 SRAM 322) in accordance with various examples. The example1300 includes a CPU core 1302 (e.g., similar to the CPU core 302described above) and a DMA engine 1304. In this example 1300, the DMAengine 1304 is similar to another of the CPU cores 102 shown in FIG. 1,which are also capable of accessing the L2 cache subsystem 102 a (e.g.,through the shared L2 cache subsystem 108). In the example 1300, the CPUcore 1302 is alternately referred to as a “producer” of data that writesto the L2 SRAM 322, while the DMA engine 1304 is alternately referred toas a “consumer” of data that reads from the L2 SRAM 322.

Both the CPU core 1302 and the DMA engine 1304 are coupled to the L2controller 320, which is in turn coupled to the L2 SRAM 322 as explainedabove. Additionally, the L2 controller 320 is coupled to a memory mapcontrol register 1306 and a memory switch control register 1308, thefunctions of which are described further below. In some examples, thecontrol registers 1306, 1308 are portions of a single control register,while in other examples the control registers 1306, 1308 are separatestructures as shown.

In some examples, the control registers 1306, 1308 are controlled bysoftware (e.g., executing on the CPU core 1302) as memory-mappedregisters. In an example, the memory map control register 1306 specifieswhether the CPU core 1302 and the DMA engine 1304 are able to view andaccess the full memory map of the L2 SRAM 322 (e.g., un-aliased mode) orare able to view and access an aliased memory map of the L2 SRAM 322(e.g., aliased mode).

If the memory map control register 1306 is set for operation in theun-aliased mode, shown in the example 1310 of L2 SRAM 322, both the CPUcore 1302 and the DMA engine 1304 are able to direct transactions tovirtual addresses in buffers IBUFLA, IBUFHA, IBUFHLB, IBUFHB. In theun-aliased mode 1310, the L2 controller 320 is configured to direct suchtransactions to the corresponding physical addresses in those samebuffers. Thus, in the un-aliased mode, the L2 controller 320 isconfigured to direct a transaction (from either CPU core 1302 or DMAengine 1304) to a virtual address in the buffer IBUFLA to thecorresponding physical address in the buffer IBUFLA in the L2 SRAM 322,and so on.

If the memory map control register 1306 is set for operation in thealiased mode, shown in the example 1312 of L2 SRAM 322, both the CPUcore 1302 and the DMA engine 1304 are only able to direct transactionsto certain virtual addresses (e.g., in buffers IBUFLA, IBUFHA in thisexample). Attempts to direct a transaction to other virtual addresses(e.g., in buffers IBUFLB, IBUFHB in this example) result in an error,explained further below. In the aliased mode 1312, the L2 controller 320is configured to direct transactions from the CPU core 1302 to a virtualaddress (e.g., in buffer IBUFLA) to a first physical address (e.g., alsoin IBUFLA) and to direct transactions from the DMA engine 1304 to thesame virtual address in buffer IBUFLA to a second, different physicaladdress (e.g., in IBUFLB). This is depicted as virtual addresses in thealiased mode 1312 of operation being mapped to different physicaladdresses 1314.

By operating the L2 controller 320 in the aliased mode, the CPU core1302 as producer writes to a certain virtual address and at the sametime the DMA engine 1304 as consumer reads from that same virtualaddress. However, due to the aliased mode of operation, the physicaladdress being produced to by the CPU core 1302 is different than thephysical address being consumed from by the DMA engine 1304. This allowsthe CPU core 1302 to produce to a physical buffer A (e.g., IBUFLA andIBUFHA) while the DMA engine 1304 consumes from a physical buffer B(e.g., IBUFLB and IBUFHB), despite both addressing the transactions tothe virtual address.

In an example, the memory switch control register 1308 specifies whichphysical address a virtual address is aliased to as a function ofwhether the CPU core 1302 and the DMA engine 1304 “owns” a certainbuffer. Ownership in this context is mutually exclusive; that is, if thememory switch control register 1308 specifies that the CPU core 1302owns buffer A (e.g., IBUFLA and IBUFHA), then the DMA engine 1304 cannotalso own buffer A. In this example, it is assumed that the owner of abuffer has its transactions aliased to physical addresses in the namedbuffer, while the non-owner of the buffer has its transactions aliasedto physical addresses in the aliased buffer. For example, if the CPUcore 1302 owns buffer A, then the L2 controller 320 is configured todirect CPU core 1302 transactions to physical addresses also in bufferA. Similarly, since the DMA engine 1304 does not own buffer A, then theL2 controller 320 is configured to direct DMA engine 1304 transactionsto physical addresses in buffer B.

By managing the memory switch control register 1308, a ping pong typeeffect is enabled that allows the CPU core 1302 and the DMA engine 1304to both believe they are producing to and consuming from a certainbuffer (e.g., by directing transactions to virtual addresses in bufferA). However, when the memory switch control register 1308 indicates thatthe CPU core 1302 is the owner of the buffer A, the CPU core 1302produces to physical addresses in the buffer A while the DMA engine 1304consumes from physical addresses in the buffer B. Subsequently (e.g.,when the CPU core 1302 is close to filling the physical addresses inbuffer A with data), the memory switch control register 1308 is updatedto indicate that the DMA engine 1304 is the owner of the buffer A. As aresult, the DMA engine 1304 begins to consume from physical addresses inthe buffer A while the CPU core 1302 begins to produce to physicaladdresses in the buffer B.

Ina more general example, the L2 SRAM 322 includes a working buffer(WBUF), a first buffer A (e.g., including IBUFLA and IBUFHA in FIG. 13),and a second buffer B (e.g., including IBUFLB and IBUFHB in FIG. 13).Because the first, second, and working buffers are portions of the L2SRAM 322, in one example a base address control register (not shown forsimplicity) is used that specifies a base address in the L2 SRAM 322 foreach of the first, second, and working buffers. In the specific exampleof FIG. 13, the base address control register specifies a base addressfor each buffer IBUFLA, IBUFHA, IBUFLB, IBUFHB, and WBUF. This allowsfurther configurability of where these buffers reside in the L2 SRAM322. In one example, the size of the IBUF buffers is fixed at 32 KB(e.g., from the specified base address) as shown, while the WBUF bufferextends to the end of the L2 SRAM 322 (from its specified base address).However, in another example, the size of the buffers is configurable.

In some examples, the L2 controller 320 is configured to indicatevarious error conditions, for example by asserting bits in an errorstatus register (e.g., in the L2 cache subsystem 306). For example, theL2 controller 320 is configured to indicate an error in response to arequest to the working buffer (WBUF) being for an address outside of anaddress range (e.g., in L2 SRAM 322) in which the various buffersreside.

In another example, the L2 controller 320 is configured to indicate anerror in response to a request to, for example, the buffer A being foran address outside of the address range for the buffer A. The addressrange for the buffer A is based on the base address for the buffer A,and the size of the buffer A, which is either fixed or configurable.

In another example, when the L2 controller 320 is operating in aliasedmode, the L2 controller 320 is configured to indicate an error inresponse to a request directed to a virtual address that maps to aphysical address in the aliased buffer. Referring back to FIG. 13 forexample, when operating in aliased mode 1312, an error is indicated ifthe CPU core 1302 or the DMA engine 1304 attempts to directly access thealiased buffer, which in this case is buffer B (e.g., IBUFLB andIBUFHB). In a general sense, in the aliased mode, accesses are permittedto virtual addresses in one buffer (e.g., buffer A) but not to virtualaddresses in the other, aliased buffer (e.g., buffer B). As a result, inaliased mode, the only way to access the physical addresses of thealiased buffer B is through the aliased mode operation of the L2controller 320.

In any of the foregoing error examples, an error clear register (e.g.,in the L2 cache subsystem 306) contains fields that correspond to fieldsin the error status register. When a field in the error clear registeris asserted, for example, the corresponding field in the error statusregister is cleared.

FIG. 14 shows a flow chart of a method 1400 for operating on the L2 SRAM322 by the L2 controller 320 in an aliased mode in accordance withvarious examples. The method 1400 begins in block 1402 with operatingthe L2 controller 320 in an aliased mode in response to a memory mapcontrol register value being asserted. The method 1400 continues inblock 1404 with the L2 controller 320 receiving a first request from afirst CPU core (e.g., CPU core 1302) directed to a virtual address(e.g., in buffer A) in a L2 memory (e.g., L2 SRAM 322) of the L2 cachesubsystem 306. The method 1400 continues in block 1406 with receiving asecond request from a second CPU core (e.g., DMA engine 1304) directedto the same virtual address in the L2 SRAM 322. As a result of the L2controller 320 operating in the aliased mode, the method 1400 continuesin block 1408 with directing the first request to a physical address Ain the L2 SRAM 322 (e.g., as shown at 1314 in FIG. 13) and in block 1410with directing the second request to a physical address B in the L2 SRAM322 (e.g., as shown at 1314 in FIG. 13).

In the foregoing discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus mean“including, but not limited to . . . ” Also, the term “couple” or“couples” means either an indirect or direct connection. Thus, if afirst device couples to a second device, that connection may be througha direct connection or through an indirect connection via other devicesand connections. Similarly, a device that is coupled between a firstcomponent or location and a second component or location may be througha direct connection or through an indirect connection via other devicesand connections. An element or feature that is “configured to” perform atask or function may be configured (e.g., programmed or structurallydesigned) at a time of manufacturing by a manufacturer to perform thefunction and/or may be configurable (or re-configurable) by a user aftermanufacturing to perform the function and/or other additional oralternative functions. The configuring may be through firmware and/orsoftware programming of the device, through a construction and/or layoutof hardware components and interconnections of the device, or acombination thereof. Additionally, uses of the phrases “ground” orsimilar in the foregoing discussion include a chassis ground, an Earthground, a floating ground, a virtual ground, a digital ground, a commonground, and/or any other form of ground connection applicable to, orsuitable for, the teachings of the present disclosure. Unless otherwisestated, “about,” “approximately,” or “substantially” preceding a valuemeans+/−10 percent of the stated value.

The above discussion is illustrative of the principles and variousembodiments of the present disclosure. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. The following claims should beinterpreted to embrace all such variations and modifications.

What is claimed is:
 1. A method, comprising: receiving, by a first stagein a pipeline, a first transaction from a previous stage in thepipeline; determining whether the first transaction comprises a highpriority transaction or a low priority transaction; in response to thefirst transaction comprising a high priority transaction, processing thehigh priority transaction by sending the high priority transaction to anoutput buffer; receiving a second transaction from the previous stage;determining whether the second transaction comprises a high prioritytransaction or a low priority transaction; in response to the secondtransaction comprising a low priority transaction, processing the lowpriority transaction by: monitoring a full signal from the output bufferwhile sending the low priority transaction to the output buffer; inresponse to the full signal being asserted and no high prioritytransaction being available from the previous stage, pausing processingof the low priority transaction; in response to the full signal beingasserted and a high priority transaction being available from theprevious stage, stopping processing of the low priority transaction andprocessing the high priority transaction; and in response to the fullsignal being de-asserted, processing the low priority transaction bysending the low priority transaction to the output buffer.
 2. The methodof claim 1, wherein stopping processing further comprises temporarilystopping processing of the low priority transaction by: storing the lowpriority transaction in a memory structure; and resuming processing ofthe low priority transaction when a high priority transaction is notavailable from the previous stage.
 3. The method of claim 1, wherein asystem data phase size is less than a transaction size, whereinprocessing the high priority transaction further comprises: sending afirst data phase of the high priority transaction to the output buffer;and in response to receiving an acknowledgement from the output buffer,sending a second data phase of the high priority transaction to theoutput buffer.
 4. The method of claim 1, wherein a system data phasesize is less than a transaction size, wherein processing the lowpriority transaction further comprises: sending a first data phase ofthe low priority transaction to the output buffer; and waiting for anacknowledgement from the output buffer and, while waiting: in responseto not receiving an acknowledgement from the output buffer, monitoringthe full signal; and in response to receiving an acknowledgement fromthe output buffer, sending a second data phase of the low prioritytransaction to the output buffer.
 5. The method of claim 1, wherein thepipeline comprises a level two (L2) cache subsystem pipeline and anoutput of the output buffer is coupled to a level three (L3) cachesubsystem.
 6. The method of claim 1, wherein receiving the first andsecond transactions from the previous stage further comprises receivingthe first and second transactions from an input buffer coupled to theprevious stage.
 7. The method of claim 6, wherein the input buffercomprises a first buffer configured to contain high prioritytransactions and a second buffer configured to contain low prioritytransactions, and determining whether the first transaction comprises ahigh priority transaction or a low priority transaction furthercomprises: determining that the first transaction is a high prioritytransaction if the first buffer is not empty; and determining that thefirst transaction is a low priority transaction if the first buffer isempty and the second buffer is not empty.
 8. A method, comprising:receiving, by a first stage in a pipeline, a first transaction from aprevious stage in a pipeline; determining whether the first transactioncomprises a high priority transaction, a medium priority transaction, ora low priority transaction; in response to the first transactioncomprising a high priority transaction, processing the high prioritytransaction by sending the high priority transaction to an outputbuffer; receiving a second transaction from the previous stage;determining whether the second transaction comprises a medium prioritytransaction or a low priority transaction; in response to the secondtransaction comprising a medium priority transaction, processing themedium priority transaction by: monitoring a full signal from the outputbuffer while sending the medium priority transaction to the outputbuffer; in response to the full signal being asserted and no highpriority transaction being available from the previous stage, pausingprocessing of the medium priority transaction; in response to the fullsignal being asserted and a high priority transaction being availablefrom the previous stage, stopping processing of the medium prioritytransaction and processing the high priority transaction; and inresponse to the full signal being de-asserted, processing the mediumpriority transaction by sending the medium priority transaction to theoutput buffer; and in response to the second transaction comprising alow priority transaction, processing the low priority transaction by:monitoring the full signal from the output buffer while sending the lowpriority transaction to the output buffer; in response to the fullsignal being asserted and no high or medium priority transaction beingavailable from the previous stage, pausing processing of the lowpriority transaction; in response to the full signal being asserted anda high or medium priority transaction being available from the previousstage, stopping processing of the low priority transaction andprocessing the high or medium priority transaction; and in response tothe full signal being de-asserted, processing the low prioritytransaction by sending the medium priority transaction to the outputbuffer.
 9. The method of claim 8, wherein stopping processing furthercomprises temporarily stopping processing of the medium or low prioritytransaction by: storing the medium or low priority transaction in amemory structure; and resuming processing of the medium or low prioritytransaction when a higher priority transaction is not available from theprevious stage.
 10. The method of claim 8, wherein a system data phasesize is less than a transaction size, wherein processing the highpriority transaction further comprises: sending a first data phase ofthe high priority transaction to the output buffer; and in response toreceiving an acknowledgement from the output buffer, sending a seconddata phase of the high priority transaction to the output buffer. 11.The method of claim 8, wherein a system data phase size is less than atransaction size, wherein processing the medium or low prioritytransaction further comprises: sending a first data phase of the mediumor low priority transaction to the output buffer; and waiting for anacknowledgement from the output buffer and, while waiting: in responseto not receiving an acknowledgement from the output buffer, monitoringthe full signal; and in response to receiving an acknowledgement fromthe output buffer, sending a second data phase of the medium or lowpriority transaction to the output buffer.
 12. The method of claim 8,wherein the pipeline comprises a level two (L2) cache subsystem pipelineand an output of the output buffer is coupled to a level three (L3)cache subsystem.
 13. The method of claim 8, wherein receiving the firstand second transactions from the previous stage further comprisesreceiving the first and second transactions from an input buffer coupledto the previous stage.
 14. The method of claim 13, wherein the inputbuffer comprises a first buffer configured to contain high prioritytransactions, a second buffer configured to contain medium prioritytransactions, and a third buffer configured to contain low prioritytransactions, and determining whether the first transaction comprises ahigh priority transaction, a medium priority transaction, or a lowpriority transaction further comprises: determining that the firsttransaction is a high priority transaction if the first buffer is notempty; determining that the first transaction is a medium prioritytransaction if the first buffer is empty and the second buffer is notempty; and determining that the first transaction is a low prioritytransaction if the first and second buffers are empty and the thirdbuffer is not empty.
 15. A level two (L2) cache subsystem, comprising: aL2 pipeline; and a state machine in the L2 pipeline, the state machineconfigured to: receive a first transaction from an input buffer coupledto a previous stage in the L2 pipeline; determine whether the firsttransaction comprises a high priority transaction, a medium prioritytransaction, or a low priority transaction; in response to the firsttransaction comprising a high priority transaction, process the highpriority transaction by sending the high priority transaction to anoutput buffer; receive a second transaction from the input buffer;determine whether the second transaction comprises a medium prioritytransaction or a low priority transaction; in response to the secondtransaction comprising a medium priority transaction, process the mediumpriority transaction, wherein when the state machine processes themedium priority transaction, the state machine is further configured to:monitor a full signal from the output buffer while the medium prioritytransaction is sent to the output buffer; in response to the full signalbeing asserted and no high priority transaction being available from theinput buffer, pause processing of the medium priority transaction; inresponse to the full signal being asserted and a high prioritytransaction being available from the input buffer, stop processing ofthe medium priority transaction and process the high prioritytransaction; and in response to the full signal being de-asserted,process the medium priority transaction by sending the medium prioritytransaction to the output buffer; and in response to the secondtransaction comprising a low priority transaction, process the lowpriority transaction, wherein when the state machine processes the lowpriority transaction, the state machine is further configured to:monitor the full signal from the output buffer while the low prioritytransaction is sent to the output buffer; in response to the full signalbeing asserted and no high or medium priority transaction beingavailable from the input buffer, pause processing of the low prioritytransaction; in response to the full signal being asserted and a high ormedium priority transaction being available from the input buffer, stopprocessing of the low priority transaction and process the high ormedium priority transaction; and in response to the full signal beingde-asserted, process the low priority transaction by sending the mediumpriority transaction to the output buffer.
 16. The L2 cache subsystem ofclaim 15, wherein when the state machine stops processing, the statemachine is further configured to temporarily stop processing of themedium or low priority transaction, and: store the medium or lowpriority transaction in a memory structure; and resume processing of themedium or low priority transaction when a higher priority transaction isnot available from the input buffer.
 17. The L2 cache subsystem of claim15, wherein a system data phase size is less than a transaction size,and when the state machine processes the high priority transaction, thestate machine is further configured to: send a first data phase of thehigh priority transaction to the output buffer; and in response toreceiving an acknowledgement from the output buffer, send a second dataphase of the high priority transaction to the output buffer.
 18. The L2cache subsystem of claim 15, wherein a system data phase size is lessthan a transaction size, and when the state machine processes the mediumor low priority transaction, the state machine is further configured to:send a first data phase of the medium or low priority transaction to theoutput buffer; and wait for an acknowledgement from the output bufferand, while waiting: in response to not receiving an acknowledgement fromthe output buffer, monitor the full signal; and in response to receivingan acknowledgement from the output buffer, send a second data phase ofthe medium or low priority transaction to the output buffer.
 19. The L2cache subsystem of claim 15, wherein an output of the output buffer iscoupled to a level three (L3) cache subsystem.
 20. The L2 cachesubsystem of claim 15, wherein the input buffer comprises a first bufferconfigured to contain high priority transactions, a second bufferconfigured to contain medium priority transactions, and a third bufferconfigured to contain low priority transactions, and when the statemachine determines whether the first transaction comprises a highpriority transaction, a medium priority transaction, or a low prioritytransaction, the state machine is further configured to: determine thatthe first transaction is a high priority transaction if the first bufferis not empty; determine that the first transaction is a medium prioritytransaction if the first buffer is empty and the second buffer is notempty; and determine that the first transaction is a low prioritytransaction if the first and second buffers are empty and the thirdbuffer is not empty.